Stabilizing treatment



Patented Nov. 22, 1938 UNITED STATES PATENT orrlca STABILIZING TREATMENT No Drawing.

Application January 21, 1036,

Serial No. 80,13.

6 Claims. (01. 148-411) cient improvement to warrant the expense. Aluminum base alloys containing from about 1 to 6.5 per cent magnesium as their chief alloying constituent are, however, well adapted to being cold worked since they develop a higher strength and hardness with less cold work than does pure aluminum or many of the aluminum base-alloys designed for fabrication in this manner.

' Aluminum-magnesium alloys containing from about 1 to 6.5 per cent magnesium in common with most metals and alloys become hardened when deformed below the recrystallization temperature. The tensile and yield strengths of the metals are also increased by the same operation. This increase in strength and hardness is very desirable in many instances since the annealed metal would be too soft to withstand loads or impacts without suffering some deformation. Within certain limits it is also possible to produce a material having specified strength and hardness values. For the most part the physical properties of cold worked metals and alloys remain substantially constant over long periods of time at ordinary temperatures since no recrystallization occurs. Cold worked aluminum-magnesium alloys, however, differ from this usual behavior in that the strength decreases somewhat over a period of a year and longer. This decrease is not of such a magnitude as to cause failure of a part in service, and in fact no difference in performance of the part can be detected under ordinary conditions.

The disadvantage of a change in strength is most apparent where articles are to be formed from the cold worked aluminum-magnesium alloys. In forming operations the machines are set to handle material of a certain strength and resistance to deformation. If the material to be formed possesses diiferent physical properties, the article will not be properly made and it will not have the intended characteristics. Where the physical properties of a material are subject to change upon standing it become difficult to estimate the exact condition at the time it is to be used. Material stored for a year differs from that stored for only six months. It thus becomes highly desirable to render the properties uniform over a period of time and to avoid the uncertainties of transition.

One of the principal objects of our invention is to stabilize the physical properties of cold I worked aluminum-magnesium alloys. Another object is to stabilize these properties in the cold worked alloys without destroying the benefit of the cold work. Still another object is to provide a treatment that can be quickly and easily applied. 20

Our invention is predicated upon the discovery that the strength and elongation of cold worked aluminum base alloys containing from about 1 to 6.5 per cent magnesium as the predominant alloying constituent can be stabilized by heating to a temperature between about 150 and 400 F. for a period of about minutes to hours. In our preferred practice we use a temperature between 275 and 350 F., and heat the metal for a period of from 3 to 10 hours. By means of this treatment we have been able to render the strength and elongation values substantially constant over a long period of time. The strength of the cold worked alloy is somewhat diminished by the stabilizing treatment but the elongation is increased. The gain in elongation, however, is especially desirable where the alloy is to be formed, since it permits more severe deformation, such as bending or drawing, than the alloy will stand immediately after being cold worked before stabilization. Although the strength is reduced by the stabilizing treatment "yet the increase brought about by the cold working is preserved in a very substantial degree. As far as we have determined our treatment does not induce any recrystallization of the alloy such as occurs upon annealing nor does the nonstabilized material, for that matter, show signs of recrystalllzing after being stored for a long so period of time.

Tensile Yield Elonga' 4 strength strength tion Pounds Pounds per I square ifi square lack Percent 47 Mg 68. 880 Kl, 700 4. b s 0 Mg 70, 760 62, 5(1) 3. 8

After being exposed to the atmosphere for one year at ordinary temperatures, the physical properties of these alloys had the following value.

Tensile Yield Elonga- Anoy strength strength tion Poundl Pounds square :53 square l Percent 4% Mg 56, oso 46, em 8. 8 0% Mg 65, 070 53, 400 10. 7

When the cold rolled alloys were stabilized by heating at 290 F. for8 hours, the following physical properties were obtained.

Yi n1 A1107 s fr e r i g tli M5 511 tgggl Poundeper Poundsper inch incl Per m 4% Mg fasln ical 13.0 mm; 66,980 40,850 10.2

Although the stabilization reduces the strength Tensile Yield Elon strength strength tlor E Pounds Pounds r mi-'3 square 1 b Percent 4 Mg 34,050 15,400 ass 0;, Mg 41,700 19,000 27.4

The decrease in yield strength and increase in elongation are to be especially noted. By comparison the stabilized alloys may be said to have retained the work hardening effect of cold rolling to a considerable degree and they can therefore be used in many places where the cold rolled product has heretofore been employed.

The increase in elongation brought about by stabilization permits a more ready forming of the alloy than when it is in the cold worked condition. It is not only easier to bend, stretch, draw or roll the stabilized product, but there is much less danger of excessive work hardening which may cause internal failure. The greater ductility of the stabilized alloys also makes it possible to subject them to more severe deformation than the non-stabilized cold worked material without excessively hardening the metal.- This increased capacity to be worked makes the alloy more useful in the production of formed articles than either the non-stabilized cold worked or annealed alloy.

The stability in physical properties of cold 5 worked aluminum-magnesium alloys which have been heated to between 212 .and 400 F. is of great advantage to both the producer and user of the alloys. The producer is enabled to provide material meeting the user's specifications 10 with reasonable certainty that no appreciable change in properties will occur in the period between production of the material and its fabrication to finished form. It has been found that the rate of change in properties fluctuates with the 15 atmospheric temperature and hence the properties can not be accurately predicted for any particular length of time under normal conditions. In the manufacture of many articles from the aforementioned alloys, the material is often stored 20 several months or longer before it is finally used. Under these conditions it becomes impossible to estimate the change which may have occurred and to make allowance for the same in the fabrication operations. The use of our stabilizing,

treatment avoids these difilculties and uncertainties.

The length of time required to stabilize an alloy is dependent on the composition, size of load or piece being treated and the temperature so to which the metal is heated. Some of the magnesium bearing alloys show a greater tendency to age soften than others when they are given the same amount of cold work, and hence these alloys should receive a longer treatment or be 3 ment of a temperature in the lower portion of 40 the stabilization range is useful, however, where a large load of metal is being treated since there is-less danger of overheating one part of the load. It is also obvious that a longer time is required to heat a larger load thana few articles. some conditions, such as heating articles in a so-called continuous furnace, it may be more economical and efllcient to employ a relatively high temperature for a short time. In any case the best combination of time and temperature 6 may be readily determined by test runs under existing operating conditions.

In addition to being able to stabilize the physical properties of alloys composed of aluminum and magnesium by our treatment, we have found 55 that cold rolled aluminum-magnesium alloys containing other elements also suffer a loss in properties upon standing and that the properties may likewise be stabilized by heating to between 150 and 400 F. for several hours. manganese, chromium, molybdenum, and titanium constitute a class of substances which enhance certain properties of the aluminum-magnesium alloys without altering their fundamental characteristics. amounts of from about 0.05 to 2 per cent and the chromium in amounts of from about 0.05 to 0.5 per cent. From about 0.02 to 0.25 per cent of molybdenum or titanium may be added. Two

or more of these elements may be employed in the same alloy if desired, and in such a case the total amount should not exceed about 1.5 per cent. Typical properties of the foregoing alloys-in the cold rolled and stabilized conditions are given below.

Under 45 The elements, so

The manganese may be used in as Cold rolled Tensile Yield Alloy strength strength Elongatwn Pounds per Pounds per square inch square inch Percent 2.5 Mg0.25 Cr 48. 730 46, 100 3. 2 1 big-1.25 M'n 44, 900 41,150 4. 3

Stabilized Tensile Yield Alloy strength strength Elongatwn Pound: per Pounds per square inch square inch Percent 2.5 Mg0.25 Cr 42, 790 37. 250 9.0 l Mg1.25 Mn 42, 3l0 37,800 7.

We are aware that low temperature treatments have been heretofore employed subsequent to solution heat treatment at elevated temperatures for the purpose of causing the precipitation of dissolved constituents and consequent hardening of the alloy. Our stabilizing treatment is designed ior cold worked alloys and not as a means of increasing the hardness of solution heat treated alloys. Our treatment, as a matter of fact, causes a reduction in hardness and an increase in elongation as compared to the opposite effect of the customary aging process.

Having described our invention and the manner in which the same is to be performed we claim:

1. A method of stabilizing the properties of age-softening cold worked aluminum base alloys containing from about 1 to 6.5 per cent magnesium, said method comprising heating the cold worked alloy to 150 to 400 F. for a period of 15 minutes to 20 hours.

mum, and titanium in amounts of about 0.05 to v 2 per cent manganese, 0.05 to 0.5 per cent chromium, 0.02 to 0.25 per cent molybdenum, and 0.02 to 0.25 per cent titanium, the total amount of said elements not exceeding about 2 per cent, said method comprising heating the cold worked alloy to about 150 to 400 F. for a period of minutes to hours.

4. A method of stabilizing the properties of age-softening cold worked aluminum base alloys containing 4 per cent magnesium, said method comprising heating the cold worked alloy to 2'75 to 350 F. for a period of 2 to 10 hours.

5. A method of stabilizing the properties of age-softening cold worked aluminum base alloys containing 2.5 per cent magnesium and 0.25 per cent chromium, said method comprising heating the cold worked alloy to 275 to 350 F. for a period of 2 to 20 hours.

6. A method of stabilizing the properties of an age-softening cold worked aluminum base alloy containing about 1 per cent magnesium and 1.25 per cent manganese, said method comprising heating the cold worked alloy to about 275 to 350 F. for a period of 2 to 10 hours.

JOSEPH A. NOCK, JR. FRED KELLER. 

